1. Field of the Invention
The present invention relates generally to a driving circuit for a stepping motor, and particularly to a driving circuit for a stepping motor comprising a rotor which is driven by a pulse train impressed on coils of magnetic poles of a stator.
2. Description of the Prior Art
Several proposals have been made on devices using a stepping motor as their driving means. Such known devices utilize digital accuracy for driving the stepping motor. For instance, one proposal has been made to configure the stepping motor is to have a large through-hole at the center into which is built an optical lens barrel structure, whereby a stop means of the optical lens is linked to be driven by the rotor of the stepping motor in digital manner, thereby giving a desired aperture number of the lens stop.
Conventional stepping motors have two known driving modes, that is a, single-phase mode and a two-phase mode.
A stepping motor operated in the single-phase driving mode has a relatively small driving torque and hence a relatively small driving speed, but it is capable of being stopped in a stable manner, since poles of a rotor face and magnetic poles of a stator face one another in the stopped state. That is, although a stepping motor driven in the single-phase mode requires a relatively long time to reach a desired angular position from an initial position because of its relatively slow response characteristic, the rotor can be controlled to stop at an accurate angular position.
On the other hand, the two-phase operation is known as having a relatively large rotation torque, so that the time required for arriving at a desired angular position is relatively short. However, when the rotor rotation of a stepping motor driven in the two-phase mode stops, two adjacent magnetic poles are excited at the same polarity. Therefore, when the exciting current ceases being applied to the magnetic pole pieces, magnetic poles of the rotor move in a manner such that they face directly in front of the magnetic poles of the stator. Accordingly, through the rotation speed is relatively large, the two-phase mode of driving a stepping motor has less accuracy in controlling stop position.
Provided that the above-mentioned stepping motors are used in a driving means for controlling the focus or other parameters of an optical lens, the use of the single-phase mode driving requires a relatively long time to arrive a desired stop position, leading to loss of opportunities to obtain good photographs or blurring of photographs which are taken. On the other hand, when the stepping motor driving the lens stop is operated in the two-phase mode of driving, a high driving speed and, hence a short stop adjusting time is obtainable. Unfortunately, the final setting of the aperture may be inaccurate, resulting in inaccurate exposures of photography. As has been elucidated, the above-mentioned driving modes of the stepping motor have respective problems, and ideal driving means to solve these problems have been looked for.